Synthesis, structure, and properties of rigid‐rod polymers with special emphasis on poly(p‐phenylene benzobisoxazole) (PBO) and poly(p‐phenylene benzobisthiazole) (PBZT) have been reviewed. Recent studies on chemical modifications and molecular simulations have also been given. After nearly 20 years of research and development, PBO fiber was commercialized in the late 1990s. However, due to processing difficulties, the concept of the so called molecular composites has not been successful. Development of the high compressive strength M5 and dihydroxy‐PBI fibers clearly suggest that there is potential for further developing properties of this class of materials. Opto‐electronic properties have also been reviewed.Synthesis of PBZT.magnified imageSynthesis of PBZT.

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Monomethyl-pendant poly(p-phenylene benzobisimidazole) (MePBI) has been spun into fiber and compared with an analogous methyl-pendant poly(p-phenylene benzobisthiazole) (MePBZT). From FTIR and WAXD, the former system has been found to exhibit intermolecular hydrogen bonding, while the latter possesses only weak van der Waals interactions. The thermomechanical properties of both systems, as examined by TGA, TMA, and DMS, are presented and discussed. A crystal structure of MePBI is proposed, which suggests intermolecular hydrogen bonding only between pairs of chains within the MePBI crystal. As compared to MePBZT and other rigid-rod polymers where the intermolecular hydrogen bond is not present, MePBI has higher compressive strength. The relatively high compressive strength of MePBI is attributed to increased intermolecular association, as opposed to differences in morphology.

Molecular mechanics and dynamics simulations have been performed on methyl‐pendant PBZT to study the effects of intermolecular crosslinking. Several possible crosslinked structures were investigated. The effect of crosslinking was found to be strongly dependent upon crosslink type and, in some instances, crosslink density. A significant axial stress is predicted to occur upon the formation of phenyl‐to‐phenyl type crosslinks. This provides a reasonable explanation for the experimental observation of transverse cracks in the skin of crosslinked, MePBZT fiber. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3057–3064, 1998

The thermomechanical behavior of several rigid-rod polymeric fibers has been investigated. Up to 300 °C, all fibers exhibited the expected axial shrinkage on heating (CTE ≈ -6 × 10 -6 °C-1 ). However, a pronounced increase in thermal contraction was observed at temperatures where polymer degradation or cleaving of the pendant group occurred. A degradation mechanism has been proposed that accounts for the evolved gases in PBZT and MePBZT. The accelerated shrinkage in these rigid-rod polymers is a result of decrease in the c-axis lattice parameter as measured via WAXD. The enhanced axial shrinkage and accompanying decrease in lattice parameter are attributed to chemical changes, and consequent cross-linking taking place within these systems which, in turn, serves to perturb the crystalline structure. The implications of this phenomenon of accelerated axial contraction, with regards to morphology, are discussed.

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